Conventionally, a reservoir tank, for example, disclosed in the Patent Literature 1, as listed below, has been known. According to the conventional reservoir tank, the discharge port and the return port are provided at a portion relatively closer to the bottom surface of the tank so that the discharge port and the return port position at a portion lower than the oil liquid surface under a normal state. This can prevent the ports from exposing to the air which exists above the oil liquid surface. Further, according to the conventional reservoir tank, a partition plate is provided for guiding air bubbles included in the oil upwardly. Thus, an air suctioning from the discharge port due to the existence of the air bubbles in the oil can be prevented.